instinctinvoluntary response by an animal to an external stimulus. The concept has come to refer to complex unlearned behaviour that is recognizable and predictable in at least one sex of a species.

Characteristics of instinctive behaviour

Heritability

Instinctive behaviour is largely heritable. Many of the activities of a species of animal are sufficiently constant and predictable to serve as specific characteristics in the same way, and often to the same degree, as do bodily structures. Examples of such heritable traits include the display movements of birds (e.g., peacocks), the web-spinning movements of spiders, the burrowing habits of marine worms, the prey-catching techniques of weasels or wolves, the food-hoarding movements of squirrels, and the browsing methods of antelope.

The genetic or inherited nature of instinctive behaviour is particularly evident in aggressive and submissive sexual behaviour and in fighting of various kinds. Other instinctive activities of this kind serve nutrition, including methods of obtaining and eating food; care of the body surface, including cleaning, grooming, and scratching movements; escape from predators, including methods of concealment, freezing or “playing dead,” and taking flight; social behaviour, including ways of responding to others both sexually and regardless of sex; and sleep, including the rhythms of rest and wakefulness and bodily positions assumed in sleep.

Many of these relatively fixed, species-characteristic types of behaviour appear to be primarily inherited; at first sight, at least, they may seem little influenced by the particular experiences of the individual animal. But much instinctive behaviour as, for example, playing or exploring, is, nevertheless, modifiable, and the detailed form of the action taken may vary according to the circumstances of the moment and the individual experience previously encountered.

Complexity of pattern

Instinctive activity is not usually a limited response to a simple stimulus but rather is a sequence of behaviour that runs a predictable course; for example, nest-building behaviour that shows a patterned sequence of acts among many birds and some fishes. Many of these actions are far from being either simple or brief. Extraordinary elaboration may be found, and, although some instinctive behaviours may be complete in seconds, others may take minutes, hours, or days.

Adaptive function

Since instinctive behaviour is assumed to be genetically based and therefore shaped by the pressures of natural selection, it follows that most of the consequences of instinctive activity contribute to the preservation of an individual or to the continuity of the species; that is, instinctive activity tends to be adaptive, contributing to the animal’s ability to reach maturity and to breed.

Stability under external change

Ideally, instinctive behaviour seems not to depend on learning or practice but to emerge in full complexity without rehearsal when appropriate stimuli or circumstances are encountered. Often, such stimuli do not guide or mold the instinctive behaviour but seem simply to trigger or release it. This characteristic gives instinct the appearance of driving the animal endogenously (from within); the quality of instinctive activity thus appears to depend only secondarily on exogenous (external) stimulation.

Experiments in which an animal is raised in a very limited environment (perhaps in isolation, as when a songbird is kept alone in a soundproofed chamber from the moment of hatching) may indicate the extent to which behaviour is spontaneous (or endogenous) or is governed (or triggered) by external circumstances. Rather than seeking to distinguish sharply between instinct and learning, however, it may be more useful to assess the degree to which a given item of behaviour is environmentally stable or unstable (labile). Indeed, aspects of behaviour of a single species may differ greatly in this respect; thus, the nesting activity, calls of alarm, and courting behaviour of birds may be extraordinarily constant under varying conditions. On the other hand, food seeking and feeding may be extremely labile (or susceptible to learning), so that geographically disparate groups of individuals belonging to the same species may have very different feeding habits.

Varieties of instinctive behaviour

No animal is ever completely isolated from some kind of environment; even in the egg or in the womb it is exposed to environmental variations just as it is after hatching or birth. There is, nevertheless, a sharp distinction between the egg or uterus phase and the free-living, active, sensing animal exposed to environmental stimuli of great diversity. In the case of the egg of an insect enclosed in a largely impervious shell or of a parasitic worm similarly isolated by an impermeable cyst wall, isolation from the environment may be so great that the animal undergoes little more than changes of temperature and oxygen supply. Thus, when such an animal emerges from its egg or cyst, the details of its structure and behaviour would seem maximally attributable to the genetic proteins in its cells; that is, its behaviour appears to be as much inherited as is its bodily structure. This sort of behaviour is environmentally so stable that it is naturally and quite reasonably given the label instinctive.

Reflex activity

A variety of what may be called simple instinctive behaviour has long been known as reflex action. When this term was introduced, it meant the simple and almost invariable response of a simple organ system (e.g., a single muscle) to a simple stimulus, such as a touch or a flash of light. In its most elementary versions, this activity has been seen as the function of an idealized mechanism that has been called the reflex arc. The primary components of the reflex arc have been identified as the sensory-nerve cell (or receptor) that receives the stimulation, in turn connecting (hence the term arc) to another nerve cell that activates the muscle cell (or effector).

Although such a reflex arc might be the simplest imaginable mechanism for inflexibly automatic behaviour, it is, as noted above, a theoretical minimum rather than an actually observed functional arrangement of cells in the body of the animal; nevertheless, a mechanism but little more complicated than this helps to account for the locomotion of such animals as millipedes. In some insects, for example, the stepping movement of one limb or muscle provides stimuli that set off another limb or muscle on a similar course of movement, providing a kind of feedback system or chain of reflex arc activities. In most cases of this sort, however, the basic physiological mechanism is more complicated than the simple arc theory would suggest. Additional nerve cells capable of communicating with other parts of the body (beyond the receptor and effector) are invariably present in reflex circuits. Such connections are what make possible the conditioning of reflex responses.

Among higher animals, and perhaps many others (such as insects), what once were thought to be chain reflexes are not systems simply linked or chained together; they are systems under the precise control of coordinated complexes of nerve cells in particular parts of the nervous system, such as the spinal cord and brain. Even without evidence of a chain of feedback (or reafferent) stimuli, performance may be smoothly integrated. This is well illustrated by the complex movements of swallowing in mammals; in the dog, for example, 11 separate muscles or muscular systems are found to discharge one after the other, precisely timed to a matter of milliseconds, and all under the control of the central nervous system (CNS: brain and spinal cord). Such complexes of precisely controlled movement, known as fixed action patterns (FAP’s), are thought to form the hard core of the inborn movement forms of instincts. When such sequences of uniform stereotyped responses seem to constitute an end point or goal-directed climax of some sort, they are known as consummatory acts.

Fixed action patterns

Some male spiders perform elaborate courtship actions that affect selectively females that are ready to respond sexually. The male, in testing for a receptive female, first stands out of reach and goes through elaborate precise gestures with limbs, pedipalps, and other body parts that are distinctively shaped or patterned in a manner characteristic of the species. Perhaps even more remarkable FAP’s are found in the displays of male fiddler crabs of the genus Uca, about 40 species of which are distributed over the Earth’s tropical ocean beaches. One of the two claws is enormously enlarged, seemingly having been evolved primarily for sexual and aggressive displays, and its ritualistic gestures are quite characteristic of the species. The timing and form of movement are so invariable from one individual to another that an expert can distinguish by this alone the species of crab, whether it comes from the shores of Panama, Tahiti, or Bali.

There must be some very precise, built-in mechanism, presumably some integration of hormonal and nervous-system controls, which ensures that each individual in a species exhibits the distinctive FAP at the right speed, amplitude, and intensity. Beyond those noted for fiddler crabs and spiders, innumerable examples of such instinctive patterns are found among the elaborate display movements of many other animals, such as insects, fishes, and birds. Among the latter three groups, these movements also are part of the species communications mechanism, serving to permit members of a species to distinguish the signals of their fellows and prospective mates from all other visual stimuli. Thus, there is a system of instinctive perceptual abilities at least as complex as the inborn tendencies to exhibit patterns of motor behaviour. Likewise, the perceptual instincts are of primary importance for the survival of the species.

Modifiable action patterns

Some types of instinctive behaviour, while showing a rigid core of fixed action pattern, are still modifiable by conditioning and other learning processes (see above). A good example is provided by the nest-building behaviour of many birds: after the breeding female has chosen a nest site, she finds and deposits sticks or twigs or pieces of grass there. A jackdaw (Corvus monedula) or rook (C. frugilegus) standing on a potential nest locality with twigs held in its beak performs a downward and sideward sweeping movement that brings the material into contact with the ledge or the branches on which the nest is to be built. The moment the twig or branch carried by the bird meets resistance, the sideways movements become more vigorous and merge into a series of quick trembling thrusts (so-called tremble shoving). When the twig is in a position that offers even more resistance, the efforts become more intense until the twig wedges fast. After this consummatory achievement the bird apparently loses interest in his activities for the moment.

An inexperienced jackdaw at first will try any objects small enough to be handled, even pieces of ice and the metal ends of small electric bulbs. None of these ever becomes lodged firmly enough by the tremble shoving to result in a stimulus that is sufficiently consummatory to ensure successful nest building. Such failure quickly extinguishes the bird’s tendency to fetch inadequate objects; equally rapid positive conditioning is effected, and the jackdaw learns to be a twig connoisseur, coming to use only those that are just right in shape and flexibility. Indeed, it has been observed that the nests of entire groups of such birds are predominantly constructed of twigs and other pieces that are taken from only one kind of tree, even though there are other building materials that are readily available.

In contrast to jackdaws, many small songbirds do apparently have an inborn tendency to select the kinds of materials that are appropriate for different phases of nest construction. This innate predilection for suitable building materials has been shown dramatically among canaries reared in man-made nests of felt. Even though female canaries thus reared have never encountered anything long and flexible before, when nest-building time comes, they can select materials appropriately. As soon as pieces of grass, bits of string, cotton, or any long flexible objects are placed in the cage, these female canaries display interest and, within seconds, carry the objects to the nest place and commence weaving movements. Once the proper state of construction has been reached, and not before, the birds display an innate tendency to line their nests with feathers, plucking out their own when no others are to be found.

A caged female canary long deprived of nesting material may take hold of one of its own feathers in its beak and, without detaching it, go through the motions of lining the nest with it again and again without, of course, actually lining the nest. Another striking example of complex nest building is found in the extraordinarily complicated movements and responses by which weaver birds build their elaborate hanging nests with such architectural features as roof, egg chamber, antechamber, and entrance tunnel. Research has identified an elaborate system of relations among external stimuli, internal hormonal conditions, neural function, and reproductive development in the behaviour of female canaries during the course of their breeding cycle. Evidence of an elaborate combination of innate physiological activity and individual experience also comes from studies concerning the development of songs and call notes among birds.

A young bird isolated from other members of its kind, or even more rigorously from all patterned auditory stimulation, produces an extremely limited, basic sound pattern. If the young bird is allowed to develop and sing along with other members of the species, however, the instinctive tendencies seem to be more fully realized through fine adjustments added by imitative learning.

With more complexity in brain structure and function as is found in many mammals, behaviour is relatively flexible, and fixed action patterns tend to be overlaid by learned patterns and to that extent obscured; nevertheless, an inexperienced brown rat deprived of nest material tends to use its own tail instead, carrying it about in behaviour that is reminiscent of the behaviour shown by a feather-deprived canary. After gathering materials, brown rats typically heap them up in a more or less circular wall and then begin to tap down and smooth the inner surface of the nest cavity. When a very inexperienced rat is offered paper strips or some other soft material for the first time, it goes into a random frenzy in which the sequence (gathering, heaping, and smoothing) is decidedly confused. Yet, each of the three phases is performed to perfection, not differing even on analysis by slow-motion films from those of an experienced rat. After having placed two or three paper strips together flat on the ground, however, the novice rat will perform heaping-up movements in the empty air above them, then apparently pat down a nest wall that is not yet in existence! Only later, after the animal has had more experience as a builder, does the rat seem to learn to inhibit the instinctive heaping and patting movements until the appropriate stages of construction have been reached.

The catalog of instinctive behaviour among animals is much richer than the few examples offered here. Despite the clear differences observable in the detailed manifestations of instincts in comparing frog, goldfish, pigeon, cat, rabbit, and man, for example, all of these behaviours may be seen to hinge on genetically transmitted physiological structures and functions. Indeed, many animal instincts may profitably be compared with some of the built-in forms of behavioral tendency among plants.

Niko an inborn impulse or motivation to action typically performed in response to specific external stimuli. Today instinct is generally described as a stereotyped, apparently unlearned, genetically determined behaviour pattern.

Defining instinct

In the past the term instinct has stood for a number of distinct conceptions about animal behaviour. For example, Alexander Jamieson, in the first volume of his A Dictionary of Mechanical Science, Arts, Manufactures, and Miscellaneous Knowledge (1829), defined the term instinct as “an appellation given to the sagacity and natural inclinations of brutes, which supplies the place of reason in mankind.”

As a rough rendering of what the term instinct means to most people, this definition still has merit. If it is taken to include the possibility that humans too can be governed by instinct, this definition is broad and vague, encompassing the variety of senses that the term has since been used to convey. However, this inclusiveness is unable to distinguish the subtle differences of meaning encompassed by the terms instinct and instinctive.

The words instinct and instinctive have borne a variety of meanings in the many different contexts in which they have been used. Their varied meanings and connotations are encountered in everyday language. For example, instinct can refer to reflexive or stereotyped behaviour, to an intuitive hunch, to a congenital aptitude or disposition, to a deep-seated impulsion (e.g., “maternal instinct”), to ways of acting that do not appear to have involved learning or experience in their development, or to knowledge that is inborn or subconsciously acquired.

The concept of instinct is complicated by the fact that it ranges across behavioral, genetic, developmental, motivational, functional, and cognitive senses. There is also a likelihood that one of these senses might be taken to entail one or more of the others. For example, evidence that a pattern of behaviour depends upon a genetic basis frequently has been assumed to imply that the pattern is unlearned. The fallacy of this reasoning should have been betrayed by the commonplace knowledge that animals can be selectively bred for attributes (e.g., trail following and sheepherding in dogs) but require extensive training for their potential usefulness to be realized. Yet the dichotomous way of thinking continues to bedevil discussion and is the basis of the recurrent dispute known as the nature-nurture controversy (heredity versus environment), which entails debate over whether behaviour, intelligence, aptitude, character, and so forth are products of genetics or of exposure to environmental factors (e.g., culture). Even British naturalist Charles Darwin argued that inheritance implied development unaffected by experience.

Darwin’s conception of motivational instinct

Darwin was well aware that the term instinct was used in several different senses. At the beginning of the chapter titled “Instinct” in his crucial work On the Origin of Species (1859), he declined to attempt to define the term:

Several distinct mental actions are commonly embraced by this term; but everyone understands what is meant, when it is said that instinct impels the cuckoo to migrate and to lay its eggs in other birds’ nests. An action, which we ourselves require experience to enable us to perform, when performed by an animal, more especially by a very young one, without experience, and when performed by many individuals in the same way, without their knowing for what purpose it is performed, is usually said to be instinctive. But I could show that none of these characters are universal.

Darwin used the word instinct in a number of different ways—to refer to what impels a bird to breed; to a disposition, such as courage or obstinacy in a dog; to selectively bred patterns of behaviour such as the tumbling movements of tumbler pigeons; to feelings such as sympathy in people; and to stereotyped actions such as those employed by honeybees when constructing the cells of a honeycomb. It is regrettable that Darwin did not make the distinctions of the meaning of instinct more explicit, for he gave powerful precedent for the indiscriminate use of the word, the ambiguity of which has repeatedly clouded and confused the understanding of behaviour.

However, there also was a positive side to Darwin’s interest in instinct. He drew attention to questions about the causal basis of actions that seemed incapable of explanation in terms of learning or cognition, and it opened up the remarkable world of animal behaviour, which seemed far removed from the world of human nature. Thus, Darwin’s legacy has stimulated the study of motivation and provided a foundation for comparative psychology and for ethology.

The following review is accordingly divided between instinct construed as impulsion, or drive; instinct viewed as inborn propensity; and instinct interpreted as behaviour.

Instinct as impulsion

Instinct as some kind of urge or driving force is exemplified by three very different kinds of motivational theory: Freudian psychoanalysis; inherent purpose, as defined by American psychologist William McDougall; and causal conceptions, as proposed by classical ethology.

Freud’s Trieb

Although Sigmund Freud, the founder of psychoanalysis, wrote in German, he used the German word Instinkt infrequently. He instead relied upon the term Trieb. While Instinkt generally refers to an automatic, unlearned, stereotyped response to a specific stimulus and hence is close to the English reflex, Trieb connotes urge, impulse, impetus, and desire—what in motivational psychology is called drive. According to the Oxford English Dictionary, this is the oldest description recorded for instinct, making it cognate with instigate.

Freud, early in his studies, took the biological view that there are two basic instinctive forces governing life: self-preservation and reproduction. This view is derived from German poet Friedrich von Schiller’s expression that “hunger and love are what moves the world,” a concept that prevailed in Freud’s Civilization and Its Discontents (1930). However, in 1915 Freud published a paper titled Instincts and Their Vicissitudes, which ultimately served as an account of how his ideas changed as psychoanalytic theory evolved. When he abandoned his attempt to explain mental life in physiological terms (The Origins of Psychoanalysis, 1954), the biological duo of drives gave way to a system in which self-preservation instinct virtually disappeared and sexual appetite dominated. Libido was supposed to consist of a kind of motivational energy internally generated with a certain strength or impetus, having the aim of achieving sexual gratification through interaction with some external object. To explain this, Freud drew on hydraulic and electrical physical analogies, as indicated in his The Neuro-Psychosis of Defence (1894):

“in mental functions something is to be distinguished—a quota of affect or a sum of excitation—which possesses all the characteristics of a quantity (though we have no means of measuring it) which is capable of increase, diminution, displacement and discharge, and which is spread over the memory-traces of ideas somewhat as an electric charge is spread over the surface of a body. This hypothesis…can be applied in the same sense as physicists apply the hypothesis of a flow of electric fluid.

The libido exerts its pressure from birth, its expression taking various forms, hence manifesting “polymorphous perversity,” as the child negotiates the passage from infancy to puberty. As a consequence, Freud recognized a number of “sexual component instincts,” which included masochism, sadism, voyeurism, and narcissism. The theory incorporating the concept of libido underwent continual revision until it reached its final form in 1923 with the publication of The Ego and the Id.

Prior to Freud’s final refinements to his concept of libido, he had developed an ancillary theory, in which aggression was admitted as a force of more or less equal standing with sexual motivation. In place of sovereign sexual libido, he was led to postulate two opposing instincts: Eros, the life instinct, and Thanatos, the death instinct (a desire to return to an inorganic state). Because Eros opposes the taking of one’s own life, which Thanatos would urge, the destructive energy of the death instinct is turned outward and expressed as aggression toward others—hence the record of carnage, cruelty, and persecution that is the burden of human history.

What consistency there is to Freud’s varying views of instinct lies in their all taking a primarily motivational form. Yet even in this there is a deep division that defies coherence. Freud vacillated between two contrasting ways of explaining behaviour: in terms of causes and in terms of intentions. At times he appealed to force and mechanism to account for compulsions, aberrations, and neurotic symptoms; at other times he called on the agency of the ego, which could devise strategies, exercise reason, and pursue purposes. In part because Freud’s ideas about instinct were varied and contained internal contradictions, later versions of psychoanalysis, such as those of German psychoanalyst Karen Horney and of British object-relations theorists, have tended to de-emphasize the role of biologically driven instinct in psychodynamics.

McDougall and behaviourism

In the early 20th century British-born American psychologist William McDougall popularized a theory of instinct based on the idea that behaviour has inherent purpose, in the sense that it is aimed at the attainment of a goal. This purposiveness was the expression of a subjectively experienced urge that stemmed from an emotional source present at birth and hence constituted an instinct. Thus, each qualitatively distinct emotional state supported a separate instinct, such as hunger, thirst, fear, aggressiveness, sexual desire, parental affection, and so on. With the publication of McDougall’s book An Introduction to Social Psychology (1908), his purposive psychology caught on to such an extent that explanation in terms of instinct became a fashion in the social sciences, including economics (as evidenced by American economist Thorstein Veblen’s The Instinct of Workmanship and the State of the Industrial Arts, 1914). However, emotions proved to be more shifting sand than bedrock for the foundation of instinct. There were no sharp criteria for distinguishing the emotions. As a result, different people produced different lists, between which there appeared to be no objective means to reach agreement. American psychologist William James had already made the point with regard to the taxonomy of the emotions that “its subdivisions are to a great extent either fictitious or unimportant….Its pretences to accuracy are a sham.”

Added to this issue over emotional boundaries was opposition from behaviourists who objected to the inclusion of the notion of subjective purposiveness in a science that was trying to be objective and to the assumption of its innate basis. In 1919 there emerged an anti-instinct revolt, which opened with the publication of the paper Are There Any Instincts? by American psychologist Knight Dunlap. Dunlap’s answer to the question proposed by his paper was negative. In it he attacked McDougall for appealing to subjective purposiveness, which was beyond the reach of observation and hence scientific validation. Other behaviourist critics brought negative evidence to bear on claims of innateness, in line with their assumption that all behaviour, apart from the simplest reflexes, is moulded by experience. By and large the friends of instinct were outdone in debate with the behaviourists, who insisted on the priority of hard facts and experiments over what they regarded as vague conjecture. However, as McDougall’s influence waned during the 1930s, a new champion for the cause of instinct—what came to be known as classical ethology—emerged in continental Europe.

Ethology and instinct

Lorenz: genetically determined behaviour

Ethology, the biological study of behaviour, rose to prominence in Europe in the late 1930s under the leadership of Austrian zoologist Konrad Lorenz and Dutch zoologist Nikolaas Tinbergen. In 1950 Lorenz stated that ethology owed its start to the discovery of a “distinct and particulate physiological process…a certain type of innate, genetically determined behaviour patterns.” He variously referred to this type of behaviour as “instinctive activity” (Instinkthandlung), “endogenous movements,” and “fixed action patterns” but maintained that it and it alone possessed all the attributes of instinct. Thus, it was supposed to be (1) stereotyped, (2) possessed by all members of at least one sex of a species, (3) innate in the sense of being genetically inherited, (4) innate in the sense of being unlearned, (5) internally patterned and controlled (once set in motion, it runs its course without further involvement of any peripheral stimulation), and (6) triggered by a specific external stimulus (“sign stimulus” or “releaser”)—this stimulus, and hence the performance of the action pattern, being the goal and terminus of variable sequences of “appetitive behaviour.” In one description, Lorenz even included the idea that an animal experiences a craving for the opportunity to perform the behaviour.

To account for the timing and duration of instinctive acts, Lorenz postulated the continuous internal generation of “action-specific energy.” This energy was expended in performance of the act and accumulated between performances, when, after reaching a certain threshold level, it drives the relevant appetitive behaviour. This motivational model recalls the analogies of Freud, but it bore little comparison to what was known of how nervous systems work, and thus it drew criticism from physiologists. Even Lorenz’s ethological colleagues had problems with it. For example, Tinbergen rejected the idea that there is a distinct kind of energy for each fixed action pattern; he sought a more integrated conception of the control of behaviour.

Tinbergen: hierarchy of motivation

In The Study of Instinct, (1951) Nikolaas Tinbergen envisaged a hierarchically organized mechanism incorporating successive patterns of appetitive behaviour that end in the performance of a fixed action pattern. “Motivational impulses” that fed into the system from a central nervous source were channeled into one or another of several outlets at each level of the hierarchy, depending upon the external stimuli encountered. The stimuli determined the course of the appetitive sequence, and this sequence progressed from general behaviour to specific behaviour, culminating in the terminal act, which was supposed to “use up” the remaining motivational impulses. Hence, the fixed action pattern was described as a “consummatory act,” conflating the senses of consummate and consume. Tinbergen envisaged such a mechanism for each of the major functional categories of behaviour: foraging, reproduction, antipredator behaviour, and so on. The example that Tinbergen worked out in greatest detail is the reproductive instinct of the fish known as the three-spined stickleback (Gasterosteus aculeatus). The reproductive instinct of this fish comprises (for the male) territorial fighting, nest building, mating behaviour, and care of offspring at the first level of the hierarchy. Each of these behaviours is made up of several alternatives constituting the next level. For example, territorial fighting consists of threat display, chasing, biting, and other aggressive actions. In order for a territory-holding fish to express its reproductive instinct in fighting, it must be confronted with trespass by a rival male. The behaviours that the territory-holding fish pursues depend upon whether the rival advances, retreats, threatens, or fights back.

While the description of behaviour as a nested hierarchy of functional categories has observational backing, Tinbergen’s motivational account of it was subject to many of the same objections leveled at Lorenz’s energy model. Tinbergen’s motivational impulses could accumulate, become dammed up, overflow, and be expended in the performance of end acts. While some of this description might apply to hormonal function, none of it applies to neural transmission—unquestionably the main means of behavioral control. Behavioral observation also posed problems. For example, many supposedly instinctive action sequences turned out to be terminated by the situation, whether it was finding a hiding place, sitting on eggs, or adjusting body temperature, rather than by the expenditure of energy in the performance of a stereotyped fixed action pattern.

In the late 1950s many ethologists insisted that the instinct concept be described in empirical terms. One result of this tough-minded approach was a loss of confidence in the adequacy of drive theories of the kinds proposed by Lorenz and Tinbergen to provide accurate accounts of behavioral control. Demands for overt quantitative indices of a drive state produced several alternative means of measurement; however, as in the behaviouristic context referred to above, these often failed to coincide with one another, implying that there must be more than a single unitary force at work. In the 1960s British zoologist Robert Hinde drew on many examples to support the view that the causal basis of behaviour is dispersed among numerous variables affecting sensory, central, and motor pathways. Since then motivational models in ethology have moved away from conceptions of energy generation and toward other ways of thinking. One idea introduced at this time was decision theory, according to which an animal selects among alternative courses of action in accordance with assessments of present and past costs and benefits in a given situation.

Instinct as innateness

Whether instinct is construed as an urge or drive or as a way of behaving, genetic inheritance is usually either explicitly asserted or implicitly assumed. Indeed, there are contexts in which a claim of genetic a priori (being independent of experience) is virtually the sole point of appeal to instinct.

Language serves as a useful example. In the 1950s American linguist Noam Chomsky forcefully argued that, although children pick up the language and dialect they hear spoken around them, they must do so on the basis of prior grammatical knowledge, constituting a “language acquisition device” that they are presumably born with. This, according to Chomsky, is implied by the existence of universal constraints on syntactic structure, in which “the poverty of the stimulus” acts as a means of accounting for the spectacular blossoming of vocabulary and syntactic command around age 2 and for the restriction of language acquisition to a “sensitive period” ending before age 10. Belief in a preformed grammatical device, for which heard speech provides input for the generation of linguistic competence and performance, has spread among linguists. For example, Canadian American psychologist Steven Pinker, in his The Language Instinct (1994), gave the theory an evolutionary interpretation.

However, the language instinct theory remains controversial. Among those who oppose the theory are researchers who used computer modeling to determine whether a “neural” network could simulate the course of language development of a human child. They claimed that all the features that the language instinct was invoked to explain can arise in a “neural network” in the absence of any predesigned structure of rules or grammar. The constraints on syntax and timing are emergent from self-organization within the network as it progressively adjusts connection strengths with repeated passage from input to output with reentrant feedback. However, the computer studies leave the issues of linguistic development unsettled. Thus, language instinct, similar to all other forms of instinct, can be described variously as encompassing genetically inherited, unlearned, reflexlike properties, driven from within, or as involving notions of impulsion, knowledge, and understanding.

Pinker’s book on the language instinct and his later book, How the Mind Works (1997), belong to a field known as evolutionary psychology. This is a movement that views the human mind as designed by natural selection. As a result, it opposes the standard social science model, according to which the mind is a general-purpose cognitive device shaped virtually in its entirety by cultural influence. In contrast, evolutionary psychology argues that the mind consists of a number of functionally specialized modules. This organization acknowledges the existence of selection pressures that acted on human Paleolithic ancestors living in small hunter-gatherer societies. Although the term instinct is lightly used in the writings of evolutionary psychologists, it is pervasively implicit in the sense of inborn propensity or innate structure.

Because evolutionary psychology can be viewed as genetic reductionism that ignores the intricacies of individual development, it is vulnerable to the kinds of criticism that comparable nativist views such as that of Lorenz have been subject to in the past. There is plenty of evidence that genes can influence behaviour. However, the question of how they do so is at issue. In the past this question either was not addressed or was assumed to involve specification of developmental outcome without any contribution from interaction with contextual factors, such as might result from experience. However, this assumption fails to acknowledge the complexity of the developmental process. At all levels—from the gene in its matrix of microcellular structure to the grown organism in its physical and social environments—interaction is the rule, which can be revealed only by developmental study. Evidence of genetic basis, which has been furnished by selective breeding and by functional correlation implying natural selection, is silent about how a behavioral trait is realized in individual ontogeny (the development of an organism from conception to adulthood).

Instinct as behaviour

Behaviour patterns regarded as instinctive range from simple reflexes to complex sequences of actions covering extended amounts of time. Occurrence can be spontaneous or selective in response to external stimuli. For example, the territorial defense behaviour of three-spined sticklebacks and of European robins can be triggered by simple patches of red, despite the fact that these animals have visual capacities comparable to that of humans.

Selective responsiveness can sometimes lead to “miscarriages” of performance. For example, Tinbergen observed sticklebacks aggressively displaying to a red postal van 30 metres (100 feet) away, visible to the fish through a window. It has also been reported that a cardinal (Cardinalis cardinalis) responded to the gaping of goldfish by regurgitating food to them as though the fish were the cardinal’s own chicks. Experiments have shown that nature can sometimes be improved upon when it comes to the stimuli evoking or guiding instinctive behaviour. For example, a nesting herring gull or oystercatcher that is offered a choice between its own clutch of eggs and a dummy egg five times the size of a real egg will select the dummy egg, even though the bird cannot sit on the egg in the normal way. In a similar example, a black-headed gull will choose a clutch of five eggs in preference to its own clutch of three, with comparable results. Such artifacts are known as supernormal stimuli. However, the kind of responsiveness they reveal has sometimes been exploited in nature, as in the case of cuckoo chicks, whose large size relative to its parasitized hosts’ own chicks elicits more than the usual amount of provisioning behaviour.

A frequently cited example from the simpler end of the range of instinctive behaviour is the food-begging behaviour of newly hatched herring gull chicks. Hungry chicks peck repeatedly at the tip of a parent’s bill until the parent gull regurgitates food. When satiated, the chicks cease pecking until the need for food arises anew. Experiments by Tinbergen and others demonstrated that the red spot at the end of the adult gull’s bill stimulates and guides a chick’s pecking and that the degree of contrast between the spot and the rest of the bill, as well as the thinness of the bill and the way in which it is moved, also contribute to the strength of the response. Indeed, Tinbergen found that a thin red rod with three white rings near its tip produced a stronger response than a real gull’s bill, thus constituting a supernormal stimulus. However, experience can change stimulus selectivity if response to suboptimal stimuli is rewarded with food. In addition, the form of the food-begging behaviour undergoes subtle modification, including a shift in the target from the tip to the base of the parent’s bill, as the chick ages.

Even more fixed in form and sensory selectivity is the prey-catching ability of frogs and toads. A hungry animal positions itself where insects or worms are likely to occur and responds to the appearance of these organisms by approach and orientation movements until the object in view is centred in the visual field and close enough to be caught. This triggers tongue-flicking, which is a ballistic response—once released, it runs its course without possibility of adjustment should the prey make a move. The prey-catching ability of frogs and toads has been studied from both behavioral and neurophysiological perspectives. These studies have shown that prey catching is guided and culminated by “bug-detector” cells in the visual system, in which neurons are tuned to moving spots that cross the retinal areas from which the neurons receive input. In the 1980s German neurophysiologist Jörg-Peter Ewert elegantly traced pattern recognition and motor command circuitry and determined how these two systems interface in the brain to effect prey detection and capture in the toad Bufo bufo. Ewert also determined that there is minimal involvement of experience or environmental input in the ontogenetic assembly (origin and development) of this mechanism. It is a model of what the term instinct generally denotes.

Another example of behaviour based on predetermined mechanisms is the nest hygiene behaviour of the honeybee. When a bee dies in a hive, a worker bee who discovers the corpse will pick it up and carry it outside, essentially throwing it away. While this appears to be an intelligent response to the possibility of contamination in the hive, it is in fact a blindly programmed reaction to the presence of a chemical, oleic acid, exuded by dead bees. Dabbing a live bee with this substance results in its being treated as though it were dead. In the case of dead larvae in hive cells, genetic study has shown that the hygienic behaviour depends on two genes on different chromosomes. One gene affects behaviour associated with uncapping of the hive cell, and the other gene affects behaviour associated with the removal of dead bodies. Of such functionally adaptive behaviour it might be said that instinct masquerades as intellect.

This observation is impressively illustrated by hive-building behaviour in bees. Hive construction is emergent from a number of simple reactions constrained by the geometry of the spaces within which the insects work. Not so amenable to reductive explanation are the vastly more elaborate structures fabricated by ants and termites. For example, in northern Australia there is a species of termite that builds an insect version of a skyscraper—a slab-sided tower 15 feet (4.6 metres) high and about 10 feet (3 metres) wide, oriented so as to catch morning and afternoon sunlight in lieu of central heating. Because of the north-south orientation, this creature is called the magnetic termite, which is only partly apt, since geomagnetism is not involved in the aligning of the building as a whole, though it does guide the insects’ construction of the galleries and chambers in pitch darkness inside the structure. Still more astounding as an engineering marvel is the nest of the bellicose termite (named for the ferociousness of its soldier caste). These insects cultivate fungus gardens within the nest, which serve to process the dead wood upon which they feed. But the fungi generate heat and also affect the air quality by absorbing oxygen and releasing carbon dioxide. To keep conditions within the limits required for optimal fungal growth and for the well-being of the termite inhabitants, there is an air-conditioning system consisting of an underground basement containing an elaborate arrangement of cooling vanes connected by flues to the various compartments above. Such intricate functional design would seem to imply the existence of an overseeing architect or central planner. But there is no evidence of any single locus of intelligence or governing agency; only the myriad scurrying termites, each going about its own narrowly limited and apparently mindless business. To try to account for functional transcendence of this sort, it has been proposed that the whole colony of individual animals constitutes a supra-organism, by analogy with the way in which the cells of a multicellular body constitute an integrated individual. However, it is not understood how such wonders of instinct are achieved.

Prominent among the more spectacular examples of behaviour patterns regarded as instinctive are many that serve the function of communication, especially in contexts having to do with competition for reproductive success. Communication behaviour consists of visual, acoustic, chemical, tactile, and electrical signaling, through which one animal can influence the behaviour of another, usually of the same species. Visual and acoustic signals have received the most attention from ethologists, since these signals are the most salient to human perception. Display postures and movements occur in bizarre forms and sequences, especially in animals with advanced perceptual capacities, such as arthropods, cephalopods, and vertebrates. In many cases, behavioral evolution has been complemented by morphological change that enhances the conspicuousness and flamboyance of choreographed movements. Such morphological influences are seen in the tail of the peacock, the plume of the bird-of-paradise, and the gaudy faces of male mandrills. When performance and panoply join in extravagant displays—such as the water-skimming pas de deux of courting western grebes, the ceremonial cavorting of courting whooping cranes, and the sequence of sexual overtures that constitutes male courtship in guppies—explanation in terms of functional significance is far from obvious.

For Darwin, such behaviour and show posed a challenge to explanation in terms of natural selection. They appeared to exacerbate vulnerability to predation by being conspicuous and by hampering movement. His answer, in The Descent of Man, and Selection in Relation to Sex (1871), was “sexual selection”—enhancement of reproductive success as a consequence of behaviour that includes conspicuous courtship displays. This comes in two forms: (1) competition between members of one sex for access to the other sex, which favours anything that promotes fighting ability, including size, strength, possession of weapons of offense and defense, and effective threat displays, and (2) mate choice, in which one sex exercises discrimination in forming sexual partnerships with members of the opposite sex. It is the latter that is invoked to account for the extent of the evolution of courtship, which can be explained by two theories. According to the “good genes” hypothesis, the choosing sex (which is usually the female) selects a mate on the basis of indications of genetic quality, which the displays of courtship are assumed to manifest. One version of this argues that the degree to which the displays constitute an impediment to predator avoidance advertises superior staying power, meaning the animal survives despite the hindrance or handicap, and thus it is to be preferred over less-constrained competitors. The other theory, sometimes called the “runaway hypothesis,” is that perceptual preferences of the choosers, for certain characteristics unrelated to genetic quality in prospective mates, can drive the evolutionary exaggeration of those characteristics to greater and greater extremes. For example, the existence of supernormal stimulation supports the idea that displays of exaggerated posture or movement that show abnormally bright or enlarged patches of colour could be favoured, thereby increasing the exaggerated representation from one generation to the next. At the same time, choosers that prefer to mate with partners possessing supernormal characteristics will be more likely to produce supernormal offspring relative to their less-discriminating competition. Thus, instinctive preference and supernormality will drive each other to evolutionary extremes, in a runaway fashion, until called to a halt by other, counterbalancing forces of natural selection.

In addition to sexual selection, there are other ways in which courtship behaviour can have functional consequences affecting evolution. For example, the physiologically arousing effects of courtship behaviour can help to synchronize the reproductive states of mating animals, hence contributing to the coordination and timing of their joint enterprise. Another important function may be prevention of miscegenation, especially where closely related species coexist. Hybrid matings can be wasteful of reproductive effort, and means to avoid them have repeatedly evolved where their occurrence is a possibility. Courtship is one mechanism by which hybrid mating can be avoided, because it provides the members of one species with a way of discriminating between their own kind and members of a related species. Consistent with this are the numerous instances of “character displacement”—greater differences in the characteristics of form and behaviour between two species in regions where they overlap (sympatry) compared with regions where they live separately (allopatry). This applies to the signals involved in the early stages of pair formation.

While species-specific form and behaviour can provide for recognition among conspecifics, there are cases of exploitation of such recognition by outsiders. A famous example is the case of the “femmes fatales” among fireflies. Flashes of light by male fireflies serve as mating signals that draw the attention of females. The flash patterns are species-specific, and a female responding to a male of the same species does so by flashing in reply at a precise interval after the male’s flash. However, females of the predatory firefly genus Photuris can mimic the replies of females of the genus Photinus. Thus, the predatory females lure Photinus males within striking distance and so obtain a meal. This kind of interspecies deceptive signaling is known as “code breaking.” There also exists “blind” deceit; for example, orchids of the genus Ophrys have flowers mimicking the shape, colour, and sex pheromones of certain species of wasps. The plants trick male wasps into trying to copulate with the flowers, and thus they coerce the wasps into serving as pollen transporters.

The relative rigidity and species distinctness of behaviour that is regarded as instinctive has sometimes been found to be sufficient to serve taxonomic purposes. Lorenz emphasized this aspect of instinct, claiming that behaviour can be regarded in the same light as morphology in terms of judging phylogenetic relationship. His own studies of the behaviour of ducks exemplified the degree to which the distribution of variety in behavioral characteristics could correlate with taxonomic affinity. Indeed, behaviour has on occasion been found to surpass anatomy for the taxonomist. In the first issue of the journal Behaviour (1948), a paper described the use of nest-provisioning behaviour by digger wasps to distinguish between species, which were later found to have only subtle morphological differences. In fact, as early as 1778, three European warblers—the willow warbler (Phylloscopus trochilus), the chiffchaff (P. collybita), and the wood warbler (P. sibilatrix), all of which are virtually identical in appearance—were distinguished on the basis of their very different songs by English naturalist and clergyman Gilbert White. Although song acquisition in songbirds involves learning of some sort in most species, it is learning that is usually constrained to the point that there is no great distortion to regarding the behaviour as instinctive, unless the focus of interest is on development.

Instinct construed as behaviour is often taken to include large-scale and composite activities, such as migration, mating, and parental care. This categorization may serve as a loose way of conveying that these kinds of behaviours can be viewed as part of the biology of a species, but it is no ally of precision. It tends to conflate Tinbergen’s four questions of animal behaviour: motivational causation, ontogenetic development, adaptive function, and evolutionary provenance. As a consequence, behavioral scientists, if they use the word instinct at all, generally restrict its application to more specific patterns of behaviour.

Prominent historical works

An eloquent chapter with representative 19th-century views on instinct is found in the second volume of William James, The Principles of Psychology, 2 vol. (1890, reprinted 1950). Prominent among the works of classical ethology are Nikolaas Tinbergen, The Study of Instinct (1951, reissued 1989)

, is a wide-ranging survey of instinctive behaviour. Papers and books devoted to special aspects of instinctive behaviour are T.H. Bullock, “The Origins of Patterned Nervous Discharge,” Behaviour, 17:48–59 (1961); Society for Experimental Biology (Great Britain), Nervous and Hormonal Mechanisms of Integration (1966), symposium papers; Robert A. Hinde (ed.), Bird Vocalizations: Their Relation to Current Problems in Biology and Psychology (1969), and Non-Verbal Communication (1972); Konrad Lorenz, “The Innate Bases of Learning,” in Karl H. Pribram (ed.), On the Biology of Learning (1969); and Ronald J. Schusterman, Jeanette A. Thomas, and Forrest G. Wood (eds.), Dolphin Cognition and Behavior: A Comparative Approach (1986). The works by Peter Marler and William J. Hamilton III, Mechanisms of Animal Behavior (1968); and W.H. Thorpe, Learning and Instinct in Animals, new ed. (1969), are also of interest

; and William H. Thorpe, Learning and Instinct in Animals, 2nd ed. (1963). Claire H. Schiller (ed.), Instinctive Behavior: The Development of a Modern Concept (1957), includes an introduction by American psychologist Karl S. Lashley and contributions by Konrad Lorenz and Nikolaas Tinbergen.

Psychology and ethology

Margaret A. Boden, Purposive Explanation in Psychology (1972), includes a discussion of William McDougall and his dispute with behaviourism. Frank J. Sulloway, Freud, Biologist of the Mind: Beyond the Psychoanalytic Legend (1992), is notable for its biological perspective on Sigmund Freud’s notions of instinct. Robert A. Hinde, Animal Behaviour: A Synthesis of Ethology and Comparative Psychology, 2nd ed. (1970), discusses the problem of instinct as unitary drive. Control theory and decision theory as alternatives to energy and drive models of motivation are covered in David McFarland, Animal Behavior: Psychobiology, Ethology, and Evolution, 3rd ed. (1998); and Frederick Toates, Animal Behaviour: A Systems Approach (1980). Neurophysiology and the causal control of instinctive behaviour are discussed in Jörg-Peter Ewert, Neuroethology: An Introduction to the Neurophysiological Fundamentals of Behaviour (1980).

Issues in behavioral development with relevance to instinct are discussed in Gilbert Gottlieb, Individual Development and Evolution: The Genesis of Novel Behavior (1992); and George F. Michel and Celia L. Moore, Developmental Psychobiology: An Interdisciplinary Science (1995).

An introduction to evolutionary psychology is Jerome H. Barkow, Leda Cosmides, and John Tooby, The Adapted Mind: Evolutionary Psychology and the Generation of Culture (1992). The innate basis of language acquisition is covered in Steven Pinker, The Language Instinct (1994).